A piezoelectric transducer is very often used to cause a mechanical element to move over a short and well-defined distance in response to a control signal.
It is not possible to mention here all of the mechanical elements that can be moved in this way. Suffice it to mention, by way of non limitative example, the diaphragm of a micro-pump intended to inject into a patient a medicament at a very small and precisely set rate, or the tool holder of a machine intended to machine a workpiece with great accuracy.
Piezoelectric transducers have two electrodes that are deposited on a body of piezoelectric material which becomes deformed in response to the application of a control voltage across the electrodes to cause the mechanical element to which it is coupled to move.
The shape and size of the body of piezoelectric material, the arrangement of the electrodes on the body and the manner in which the latter is coupled to the mechanical element having to be moved will not be described here as they depend on the use of the transducer and may therefore vary widely.
The control voltage having to be applied to these transducers to make them work, i.e. for their body of piezoelectric material to become deformed in the required manner, is generally greater than 50 volts, and very often much greater than 100 volts.
Now, portable apparatuses fitted with such transducers obviously have to be supplied by an autonomous supply source, which may only consist of primary batteries or recharge able accumulators. In many cases, the available space in such apparatuses is so limited that this source may only consist of one single primary or rechargeable cell.
Thus, in the great majority of cases, the voltage supplied by the electrical power source of portable apparatuses equipped with a piezoelectric transducer is far too low to energize the latter. These portable apparatuses must therefore include a control device able to produce the high voltage needed for the transducer's operation off the low voltage produced by their supply source, and to apply this high voltage to the transducer when the latter is required to operate.
FIG. 1 diagrammatically illustrates a known device for use in such a portable apparatus.
This device, generally referenced 1, comprises a piezoelectric transducer 2, an electrical power supply source 3 consisting for instance of a primary cell supplying a voltage of around 1.5 V, a voltage increasing circuit 4 for supplying, from this relatively low voltage, the high control voltage required for the operation of transducer 2, and a switching circuit 5 for applying the control voltage to transducer 2 or for short-circuiting the latter depending on whether or not it needs to be deformed.
The positive terminal and the negative terminal of cell 3 are respectively referenced 3a and 3b, and in the remainder of this description it will be assumed that the potential of terminal 3b is the reference potential of device 1. All voltages mentioned hereinafter will thus be voltages that are measured with reference to terminal 3b.
The voltage increasing circuit 4 has two supply terminals 4a and 4b that are respectively connected to the positive terminal 3a and to the negative terminal 3b of cell 3, a capacitor 6 and a circuit 7 for charging capacitor 6.
Charging circuit 7 comprises a coil 8 and a diode 9 which are series-connected, in that order, across terminal 4a and one of the terminals of capacitor 6, the other terminal of capacitor 6 being connected to terminal 4b.
Charging circuit 7 also comprises a MOS transistor 10 whose source and drain are respectively connected to supply terminal 4b and to the connection point A between coil 8 and diode 9. The channel of transistor 10 is thus connected in parallel with the circuit formed by diode 9 and capacitor 6.
The gate of transistor 10 is connected to the output 11a of an oscillator circuit 11 supplied by cell 3 and which has a control input 11b whose function will be described below.
Control input 11b is connected to the common point B of two series-connected resistors 12 and 13 forming a voltage divider which is connected in parallel with capacitor 6.
Oscillator circuit 11 comprises a reference voltage source, not shown separately, which supplies a voltage Ur, and is so arranged that when its input 11b is at a lesser voltage than voltage Ur, its output 11a issues a periodic signal which alternately renders transistor 10b conductive and non- conductive, and that when its input 11b is at a voltage greater than voltage Ur, its output 11a issues a continuous signal which permanently blocks transistor 10. Oscillator circuit 11 is well-known to specialists and will not be described in greater detail here.
The values of resistors 12 and 13 are so selected that voltage Ub at point B, which is also the voltage of input 11b of oscillator 11, is equal to the abovementioned reference voltage Ur when voltage Uc across the terminals of capacitor 6, which is also the voltage supplied by circuit 4 to its output 4a, is equal to the control voltage Ut required for the operation of transducer 2.
Thus, as long as voltage Uc is less than control voltage Ut, voltage Ub is less than reference voltage Ur and transistor 10 is alternately put in its conductive state and its non- conductive state by the signal supplied by the output 11a of oscillator 11.
As long as transistor 10 is conductive, it lets a current through from terminal 3a to terminal 3b of cell 3 via coil 8.
Whenever transistor 10 becomes non-conductive, this current is diverted through diode 9 to capacitor 6 and charges the latter, due to the fact that the voltage at point A then increases until it reaches a value greater than that of voltage Uc.
Voltage Uc thus progressively increases until it exceeds voltage Ut with the result that voltage Ub becomes greater than voltage Ur. Oscillator 11 then stops being operative and transistor 10 becomes non-conductive.
This situation does not change until capacitor 6 discharges and voltage Uc drops back to a value less than voltage Ut, with the result that voltage Ub again becomes less than voltage Ur.
Oscillator 11 then starts being operative again and capacitor 6 is again charged as described above.
Voltage Uc is thus continuously substantially equal to control voltage Ut, the characteristics of the various components of circuit 4, in particular the frequency of the periodic signal issuing from the output 11a of oscillator 11 when the latter is operating and the hysteresis of the circuit which, in oscillator 11, compares voltage Ub to reference voltage Ur, being so selected that the difference between voltages Uc and Ut will always be small.
Switching circuit 5, which is intended for applying to transducer 2 the voltage Uc produced by the voltage increasing circuit 4 in the manner just described, includes a MOS transistor 14 whose source is connected to the negative terminal 3b of cell 3 and whose drain is connected to the output 4c of the voltage increasing circuit via a resistor 15. The gate of transistor 14 is connected to a terminal 5a for receiving a control signal described below.
Circuit 5 further comprises another MOS transistor 16 whose drain is directly connected to the output 4c of circuit 4 and whose source is connected firstly to the drain of transistor 14 via a diode 17 and secondly to an output terminal 5b via a resistor 18. The gate of transistor 16 is connected to the drain of transistor 14.
One of the electrodes of transducer 2, referenced 2a, is connected to the above-mentioned output terminal 5b, and the other electrode of transducer 2, referenced 2b, is connected to a second output terminal, 5c, which is itself connected to the negative terminal 3b of cell 3. This second output terminal 5c could in fact be dispensed with, the second electrode 2b of transducer 2 being then connected, for instance, directly to the terminal 3b of cell 3.
The circuit that applies to the terminal 5a of switching circuit 5 the above-mentioned control signal has not been shown as its structure depends on the nature of the apparatus of which device 1 forms part. Suffice it to note that, whatever its structure, the circuit is so designed that the above control signal selectively takes up a first state in which it renders transistor 14 nonconductive and a second state in which it renders transistor 14 conductive.
In the FIG. 1 example, the control signal may have the same potential as the negative terminal 3b of cell 3 when it is in its first state and have the same potential as the positive terminal 3a of cell 3 when it is in its second snare.
It will readily be seen that when this control signal is in its first state and the transistor 14 is blocked, transistor 16 is conductive. The voltage Uc produced by the voltage increasing circuit 4 is thus applied across the electrodes 2a and 2b of transducer 2, thereby causing the desired deformation of the transducer's body made of piezoelectric material of transducer 2 and the displacement of the mechanical element connected thereto.
It will also readily be seen that, when the control signal applied to the input 5a of switching circuit 5 is in its second state and transistor 14 is conductive, transistor 16 is blocked. Transducer 2 is therefore disconnected from voltage increasing circuit 4 and its electrodes 2a and 2b are practically short-circuited. Transducer 2 is thus in its state of rest.
Diode 17 only serves to improve the blockage of transistor 16 when transducer 14 is conductive, and resistor 17 serves to limit the current that flows either through transistor 16 at the instant when the latter is rendered conductive, or through transistor 14, also at the instant when the latter is rendered conductive.
The capacitor 6 of voltage increasing circuit 4 generally has quite a large capacity, of the order of a few microfarads. Additionally, capacitor 6 must of course also be able to withstand the high voltage produced by circuit 4. It follows that capacitor 6 is a rather bulky component that is difficult and sometimes impossible to house in the apparatus that includes device 1 when this apparatus must be portable and be of very small size. Further, capacitor 6 is a rather expensive component whose cost adversely affects the cost price of known devices such as the device 1 of FIG. 1.
Moreover, transistors 14 and 16 must also be able to withstand the high voltage produced by circuit 4 and are therefore also expensive components whose cost adversely affects the cost price of the known devices such as the FIG. 1 device.
Besides, the voltage divider formed by resistors 12 and 13 dissipates quite a large amount of electrical power because of the high voltage that is produced by circuit 4 is constantly applied thereto.
The electrical power that is dissipated by this voltage divider must of course be supplied by the cell 3, to the detriment of its lifetime.